In order to be able to incorporate solids into liquid media, high mechanical force is necessary. It is usual to use dispersants in order to lower the dispersing forces and in order to minimize the total input into the system of energy needed to deflocculate the particulate solids and hence to minimize the dispersing time. Dispersants of this kind are surface-active substances of anionic, cationic or neutral structure. These substances, in a small amount, are either added to the dispersing medium or applied directly to the solid. It is also known that, following complete de-flocculation of the agglomerated solids into primary particles, after the dispersing operation, there are also instances of re-agglomeration, thereby completely or partly nullifying the dispersing effort. As a consequence of the inadequate dispersing and/or as a result of re-agglomeration there are unwanted effects: viscosity increase in liquid systems, shade drift, losses of gloss in paints and coatings, and the reduction of mechanical strength in plastics. Different formulations of inks require the use of effective dispersants for uniformly distributing the pigments in an organic medium and for minimizing the re-agglomeration that can occur during storage: inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance and/or hue shifts. In particular in inks for ink-jet systems, particularly good dispersion of the pigment particles is required to ensure unimpeded passage of the pigment particles through the nozzles of the print-head in the ink jet printer, which are usually only a few micrometers in diameter. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles has to be avoided in the standby periods of the printer. In special applications, such as decoration of ceramic by inkjet printing, the characteristics of the dispersants are very challenging because they need stabilization of inorganic pigments with high specific weight, in concentrated formulations that must have low viscosity (i.e. 8-30 mPa·s) and small particle size (<1 μm).
A multiplicity of different substances are nowadays used as dispersants for pigments and fillers. Besides simple compounds of low molecular mass, such as lecithin, fatty acids and their salts, and alkylphenol ethoxylates, complex structures are used as dispersants. These dispersants are mainly based on polymeric structures derived from oligo- or polyamines reacted with polyesters or polyethers, or from polyurethanes and polyisocyanates.
Many polymeric dispersants contain in one part of the molecule the so-called anchor group, which adsorbs onto the pigments to be dispersed. In a spatially separated part of the molecule, polymeric dispersants have polymer chains sticking out whereby pigment particles are made compatible with the dispersion medium, i.e. stabilized. The properties of polymeric dispersants depend on both the nature of the monomers and their distribution in the polymer. Polymeric dispersants obtained by statistically polymerization of monomers or by polymerizing alternating monomers generally result in poor dispersion stability. Improvements in dispersion activity are obtained using graft copolymer dispersants.
Graft copolymer dispersants which contain a back-bone based on poly(alkylene)imine and polyester chain moieties derived from one or more hydroxycarboxylic acids or lactones thereof have been known for a long time. In EP 158 406 and EP 208 041 use is made of amino and amide-functional polymers and oligomers based on polyamines and homo-polycaprolactones, in which most of the reactive amino groups have been converted into amide groups, to provide polymers and oligomers comprising a mixture of salt and amide groups. The polyesters are not terminated or contain a terminal alkylcarbonyl-group. In this case the polyester can be obtained by polymerization of an hydroxycarboxylic acid or lactone thereof in the presence of an alkylcarboxylic acid as polymerizing terminating group to give a polyester chain having only a free carboxylic acid. These dispersants are claimed to be particularly effective in more polar medium such as ketones and esters, but they have limited performance in non-polar medium; in any case their solubility is somewhat limited particularly at temperature below the normal ambient temperature.
The dispersants reported in U.S. Pat. No. 4,224,212 are the reaction products of polyethyleneimine and a polyester terminated by a carboxylic acid group, the preferred polyester being derived from hydroxycarboxylic acids having from 8 to 14 carbon atoms between the carboxylic acid group and the hydroxy group, for example from ricinoleic acid or from 12-hydroxystearic acid. The dispersants are claimed to be effective in non-polar medium, such as aliphatic solvents, however they have limited performance in polar mediums. The properties of such dispersants are claimed to be improved by branching the alkylene group of the polyester chain (EP 958041) or by using polymerization terminating groups containing branched aliphatic chain (US 2003/181544).
EP 1911813 and EP 1911815 describe non aqueous inkjet inks in which the molar percentage of amide groups based on the total nitrogen content of the polymeric dispersant is important for the formulation of a stable inkjet ink formulation.
Polymeric dispersants based on polyisocyanates or polyurethanes are also widely described, for example in EP 154 678, EP 438 836, WO 2004/104064, WO 2008/070601 and WO 2009/148836.
Few patents discloses the combination of polyalkylenimine reacted with epoxides to obtain hydroxyalkylated polyalkylenimine and further grafted with other polymers. US 2009/0131555 describes a dispersant based on a polyalkylenoxyamine linked to a poly(lower alkylene)imine by a dibasic acid. This dispersant is claimed to be suitable to disperse particulate solid in polar organic media and water. U.S. Pat. No. 7,214,653 discloses products based on a polyalkylenimine alkyleneoxide copolymer wherein the polyalkyleneoxide is constituted on average of at least of 2 oxyalkylene repeating unit (preferentially oxyethylene) having different terminal groups to modulate the characteristics of the product. The products of the invention can be suitably used as builders for detergent, as detergents, as water treatment agents and as dispersants.
At last, JP 248029 and JP 6312335 describe dispersing agents obtained by reacting polyethylenimines with epoxides to partially block the amine groups in the polyethylenimine and then by reacting the obtained products with polyesters from hydroxystearic acid or copolymers based on ε-caprolactone and hydroxystearic acid, terminated with a carboxylic acid group. Up to 1 part by weight of epoxide may be reacted with 1 part by weight of polyethyleneimine, which means that the equivalent of epoxides are lower than the equivalent of nitrogen atoms in the polyethylenimine; the polyesters are said to make an amide or salt linkage with the polyethylenimine. The reaction of the polyethylenimine with epoxides is said to avoid the release of odor from the decomposition of the primary amine groups of the polyethylenimine or from the low molecular weight oligoamines present in polyethyleneimine.
In general, in dispersants based on polyalkyleneimine, most of the amine groups are amidated or salified by reaction with polyesters; fewer products are based on polyalkyleneimine linked to aminopolyethers (in general from ethylene and/or propylene oxide) by an anhydride (EP 1648970).
Due to the transformation in amides or salts, the amino groups of the polyalkyleneimine lose basicity.
The dispersants presented above often represent only partial solutions to the problems of dispersion and stabilization. In view of the multiplicity of organic and inorganic pigments and filling materials that are used today, sufficient stabilization of the particulate solids to be dispersed, by desorption-stable occupancy of the surface, is not adequately ensured. This is particularly evident in special applications, such as in decoration of ceramic by inkjet printing, where the stabilization of inorganic pigments with high specific weight in concentrated formulations that must have low viscosity and small particle size, is required.
The present invention is therefore based on novel structures designed to eliminate the above-described disadvantages of known dispersants, in other words on developing dispersing additives which, while effectively stabilizing pigments or fillers, lower the millbase viscosity of inks, paints, pastes or plastics formulations to an extent such that processing with a high degree of filling is possible.
Surprisingly, it has become apparent that a marked improvement in the stabilization of pigments or filler particles in inks, pigment pastes, binders, or plastics formulations can be achieved when the addition compounds prepared from polyalkyleneimines, reacted with about one mole of alkylene oxide per nitrogen atom and further reacted with polyesters, are used. These dispersants strongly low the energy required for preparing millbases with suitable particle size for inkjet inks, reducing viscosity during dispersing and milling. Moreover they keep an excellent long term stability, making it possible to prepare stable formulations having a high solids fraction and low viscosity. This class of dispersant has characteristics very favorable to the preparation of ink-jet inks for ceramic decoration.